Azure Cosmos DB for MongoDB: Error 261 (TooManyLogicalSessions) During Initial List Collections Call

Question

Azure Cosmos DB for MongoDB: Error 261 (TooManyLogicalSessions) During Initial List Collections Call

Felix 0

Problem description

I am encountering error 261, labeled as 'TooManyLogicalSessions', when attempting to connect to my Azure Cosmos DB for MongoDB (vCore) resource. This issue occurs during the first call to 'list collections' from a .NET Core application using SDK version 10.x.x. I expected to access the database for development purposes, but the error appears immediately on the initial list collections attempt. The problem started around 2026-09-11T03:04:26Z. I have not observed any open connections at that time and did not find specific guidance for error 261 in the available self-help materials.

Environment

Service: Azure Cosmos DB for MongoDB (vCore); Client: .NET Core application; SDK version: 10.x.x; Operation: listing collections; No specific region, tier/SKU, or resource names provided.

What I've already tried

I have not tried any troubleshooting steps yet.

Current status

I am seeking guidance on resolving the error 261 during the initial list collections call, understanding whether this is related to session limits or configuration issues, and how to mitigate or prevent this error in my development environment. However, I suspect it is a bug in the database.

SIVASANKAR YEDDULA 250 Reputation points Microsoft External Staff Moderator

2026-09-11T04:48:16.83+00:00

Hi @Felix ,

Thank you for reaching out to the Microsoft Q&A forum.

To investigate this issue further, I have sent you a private message. Could you please provide the following details through the private message?

Error or failure screenshots

Your email address

Time zone

Your availability for a Microsoft Teams meeting

These details will help us perform a deeper investigation and engage the appropriate engineering team if required.

Once you've shared the requested information, we will continue our analysis and provide further guidance.
Felix 0 Reputation points

2026-09-11T07:18:58.95+00:00

This database is just used in a development environment, which hasn't been running overnight. There shouldn't be any sessions at all. This happened on the first request after spinning up our dev environment.
SIVASANKAR YEDDULA 250 Reputation points Microsoft External Staff Moderator

2026-09-12T09:38:12.35+00:00
Hello Felix,

Thank you for the additional information.

We understand that this is a development environment and that the error occurred on the very first request after the environment was started. Given that the application was not running overnight and there should be minimal or no active sessions, the behavior does not immediately align with a typical logical session exhaustion scenario.
Recommended Mitigation

While the investigation is ongoing, we recommend the following:

Ensure that the application reuses a single MongoClient instance and avoids creating multiple client connections unnecessarily.

Restart the application or development environment to clear any stale client connections or cached sessions.

Review the application's connection pooling and session management implementation to verify that sessions are being reused appropriately.

Capture the complete exception details, stack trace, Activity IDs, and timestamps when the issue occurs.

Confirm whether the issue reproduces consistently after a clean startup of the development environment.

Current Assessment

Since the failure occurs during the initial List Collections operation and appears immediately after startup, the issue may require additional backend validation to determine whether it is related to:

Logical session allocation within the service.

Stale session tracking.

Application connection management behavior.

A potential service-side condition affecting session availability.

At this stage, we cannot conclusively determine that the issue is caused by the application. Backend investigation is still required to validate the actual logical session utilization and identify the source of the TooManyLogicalSessions (261) error.

We will continue the investigation and share further findings as soon as additional information becomes available.

Thank you for your patience and cooperation.

Kind Regards,

Sivasankar Yeddula,
Microsoft Azure Support.
SIVASANKAR YEDDULA 250 Reputation points Microsoft External Staff Moderator

2026-09-17T05:02:59.71+00:00

Hi @Felix ,

I hope the information provided has been helpful. If you have any further questions or need additional assistance, please let us know in the comments, and we will be glad to help. If this answer helps, please consider clicking Accept Answer so that other community members with similar questions can benefit as well.
SIVASANKAR YEDDULA 250 Reputation points Microsoft External Staff Moderator

2026-09-18T07:29:51.5+00:00

Hi @Felix ,

I hope the information provided has been helpful. If you have any further questions or need additional assistance, please let us know in the comments, and we will be glad to help. If this answer helps, please consider clicking Accept Answer so that other community members with similar questions can benefit as well.
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This comment has been deleted due to a violation of our Code of Conduct. The comment was manually reported or identified through automated detection before action was taken. Please refer to our Code of Conduct for more information.
SIVASANKAR YEDDULA 250 Reputation points Microsoft External Staff Moderator

2026-09-26T04:47:22.83+00:00
Hi @Felix,

Thank you for the additional clarification.

Based on the information provided, Error 261 (TooManyLogicalSessions) is being returned during the initial List Collections operation against Azure Cosmos DB for MongoDB (vCore). Since this is a development environment that was not running overnight and the error occurred on the first request after startup, the reported behavior does not immediately align with a typical scenario where the application has gradually exhausted logical sessions.

At the same time, the error alone is not sufficient to conclude that this is a database bug. We need to distinguish between client-side session/connection behavior and the session state being reported by the service.
Recommended mitigation and validation

Reuse a single MongoClient instance

Verify that the application creates and reuses a long-lived MongoClient instead of creating a new client for each request or operation.

Review connection and session lifecycle

Check whether application startup, dependency injection configuration, retry logic, background services, or health checks are creating additional clients or sessions.

Perform a clean-start reproduction

Restart the development application/environment and reproduce the issue from a clean state. Confirm whether Error 261 occurs consistently on the first List Collections request.

Capture the complete exception

Please retain the full exception text, stack trace, timestamp, and any Activity ID/correlation information returned with the failed operation. These details are important for correlating the client request with service-side telemetry.

Compare with an isolated client test

If possible, test connectivity against the same database using a minimal application or another supported MongoDB client. This can help isolate whether the behavior follows the database resource or the current application/client configuration.
Current assessment

Because the error is reportedly occurring on the first operation after an idle period, I would not assume that this is normal client-side logical-session exhaustion.

The investigation should validate whether the failure is associated with:

application connection/session management;

persisted or stale session state;

logical-session allocation reported by the service; or

another service-side condition affecting session availability.

Until the backend session information is correlated with the timestamp of the failure, there is not enough evidence to classify the behavior as either an application issue or an Azure Cosmos DB defect.

Therefore, I recommend keeping the RCA under investigation while completing the client-side validation above and correlating the failing request with the Azure Cosmos DB for MongoDB (vCore) service telemetry.

If the issue reproduces again, please capture the exact UTC timestamp together with the full exception and available correlation/Activity ID information. This will provide the strongest evidence for further service-side investigation.

Hope this helps clarify the next troubleshooting steps.
SIVASANKAR YEDDULA 250 Reputation points Microsoft External Staff Moderator

2026-10-02T09:39:25.5133333+00:00

Hi @Felix ,

I hope the information provided has been helpful. If you have any further questions or need additional assistance, please let us know in the comments, and we will be glad to help. If this answer helps, please consider clicking Accept Answer so that other community members with similar questions can benefit as well.

1 answer

Your answer

SIVASANKAR YEDDULA 250 Reputation points Microsoft External Staff Moderator

2026-09-11T04:48:16.83+00:00

Hi @Felix ,

Thank you for reaching out to the Microsoft Q&A forum.

To investigate this issue further, I have sent you a private message. Could you please provide the following details through the private message?

Error or failure screenshots

Your email address

Time zone

Your availability for a Microsoft Teams meeting

These details will help us perform a deeper investigation and engage the appropriate engineering team if required.

Once you've shared the requested information, we will continue our analysis and provide further guidance.
Felix 0 Reputation points

2026-09-11T07:18:58.95+00:00

This database is just used in a development environment, which hasn't been running overnight. There shouldn't be any sessions at all. This happened on the first request after spinning up our dev environment.
SIVASANKAR YEDDULA 250 Reputation points Microsoft External Staff Moderator

2026-09-17T05:02:59.71+00:00

Hi @Felix ,

I hope the information provided has been helpful. If you have any further questions or need additional assistance, please let us know in the comments, and we will be glad to help. If this answer helps, please consider clicking Accept Answer so that other community members with similar questions can benefit as well.
SIVASANKAR YEDDULA 250 Reputation points Microsoft External Staff Moderator

2026-09-18T07:29:51.5+00:00

Hi @Felix ,

I hope the information provided has been helpful. If you have any further questions or need additional assistance, please let us know in the comments, and we will be glad to help. If this answer helps, please consider clicking Accept Answer so that other community members with similar questions can benefit as well.
Deleted

This comment has been deleted due to a violation of our Code of Conduct. The comment was manually reported or identified through automated detection before action was taken. Please refer to our Code of Conduct for more information.
SIVASANKAR YEDDULA 250 Reputation points Microsoft External Staff Moderator

2026-10-02T09:39:25.5133333+00:00

Hi @Felix ,

I hope the information provided has been helpful. If you have any further questions or need additional assistance, please let us know in the comments, and we will be glad to help. If this answer helps, please consider clicking Accept Answer so that other community members with similar questions can benefit as well.

Answer 1

Hi @Felix ,

Thank you for contacting Microsoft Azure Support.

We have reviewed the information provided regarding the Error 261 (TooManyLogicalSessions) encountered when accessing your Azure Cosmos DB for MongoDB (vCore) database.

As part of our investigation, we have completed the following actions:

Reviewed the reported error and environment details.

Confirmed that the failure occurs during the initial List Collections operation and returns Error 261 (TooManyLogicalSessions).

Recommended validating that the application reuses a single MongoClient instance instead of creating multiple client instances.

Suggested restarting the application/service to clear any stale or inactive client sessions.

Requested the complete exception details, stack trace, and Activity IDs (if available) to assist with further correlation.

Recommended reviewing the application's connection and session management implementation.

Initiated an investigation of the Azure Cosmos DB for MongoDB (vCore) cluster to validate logical session utilization and identify any abnormal session consumption patterns.

If service-side session exhaustion is confirmed, additional backend analysis will be performed to identify the source of excessive logical session usage and determine appropriate remediation steps.

Current Status

The issue remains under investigation. Additional backend validation is required to determine whether the TooManyLogicalSessions (261) error is caused by logical session exhaustion, stale session allocation, application connection management behavior, or a service-side condition affecting session availability.

We will share further findings as soon as they become available.

Thank you for your patience and cooperation.

Kind regards,

Sivasankar Yeddula
Microsoft Azure Support.